The direct heating up of a fluid which is already hot comes up against the difficulty of using, as a fuel, a liquid (e.g., of the fuel-oil type for feeding a burner whose ducts may be subjected to the temperature of the said fluid, which temperature may be in the order of a few hundreds of degrees and entail a danger of excessive vaporization or even of coking of the said liquid fuel. A known solution for overcoming the above noted difficulty consists of placing the combustible liquid feed pipe inside the said combustible gas duct and of feeding a mixed injector, that is, one which may operate either with gas or with a combustible liquid. This known solution has two great disadvantages: (1) on the one hand, the gas pipe on which the injectors are fixed is brought to a temperature which is clearly higher than the liquid pipe placed inside it, so that the coupling of the said liquid pipe to the corresponding injector can be formed only through a line having multiple spirals necessary for absorbing the differences in expansion at its ends and is thus liable to become blocked up; (2) on the other hand, the use of a mixed injector which is designed for alternating use of the two fuels cancels all versatility in the use of the fuels, since the fuel of one of them is exclusive of that of the other.